How and why do many dogs interrupt a futbol match?

I see so many videos of dogs going on the pitch while a futbol match is in play, either in South America or Europe in the stadium. I don’t understand how this could happen and why this happens so very often. Seems like these places have very lax stadium security but I would have assumed these places have closed the gates to prevent riots or human violence, so even dogs wouldn’t be able to pass through. Actually I’m even wondering if these leagues/stadiums release the dogs on the pitch purposes for marketing reasons…

To be clear I absolutely love dogs but I fear that some ill tempered player might take it out on these dogs when a game isn’t going their way, for example. I’ve seen one guy literally grab a small dog by its neck and toss it out… So even if it’s cute I’d rather they stay off the field.